| I would like to quote the
following, from a translation of a quotation by the eminent German
biological scientist, Albert Pfeffer which he made in 1897, concerning
intercellular communication in plants. "General physiological considerations on the establishment and maintenance of correlative harmony, in short how the chain of stimulus transmission, renders a continuity of the living substance so essential that it would be necessary to propose it, even if it were not already discovered. However, it is quite likely that the connections would also be utilized in the transport of substances and even, in particular cases, principally or solely for this purpose." In the same year, Eduard Tangl observed intercellular strands between cotyledonary cells of Strychnos nux-vomica which he interpreted as protoplasmic contacts. He is accredited as having launched the visionary concept that cell-cell communication integrates the functioning of plant tissues. Thus ever before plasmodesmata had been recognized, Pfeffer and Tangl realized that intercellular communication between living plant cells was of necessity vital to the well being , not only of the living cells, but of the plant as well. He realized that there was an absolute necessity that living cells needed to somehow communicate ‘information’ to one another. He realized that intercellular communication was somehow, a central issue in the day to day life of plants. We might begin this discussion with a few questions that need to be addressed.
In essence, plasmodesmata are:-
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Primary plasmodesmata are generally formed during the early stages of cell wall formation, that is, they exist at the time when cytokinesis results in the formation first, of a phragmoplast, and then of the developing primary cellulosic cell wall. They begin as strands of endoplasmic reticulum -- some authors state these are ‘trapped’ by the phragmoplast, but this is not necessarily the case. None the less, the ER crosses the phragmoplast and developing cell wall. One must assume that there is some ability to direct cell to cell, and nucleus to nucleus communication, from a very early stage in the development of these structures. Under some circumstances, there is a possibility that modifications of the cell wall, may give rise to the formation of secondary plasmodesmata. Secondary plasmodesmata are formed for example, at the site of the scion in a graft union, where the grafted material has to form lines of communication between the differing cell lines, to allow the passage of small molecules and other information signals from the one cell to the other. Recently, there has been a great deal of discussion concerning the structural modification of plasmodesmata, where virons and virus material traverses cell-cell domains. In particular, at such sites, there is evidence that the plasmodesmata may become modified, and branched plasmodesmata are argued, as being evidence of virion and virus passage. However, the debate rages on, as to whether these branched plasmodesmata, really constitute secondary plasmodesmata, as they (a) were there before the virus infection and (b) do not interconnect ‘unlike’ tissues -- i.e., the cell connections existed before the infection and have therefore only been modified. Click HERE to go back to the INDEX Much has been written about the potential mode of action/function of plasmodesmata. What we do know, is that they are capable of allowing passage of relatively small molecules (usually molecules with a molecular mass of less that 800 Daltons is quoted). We do know that they are involved in molecular trafficking, and we do know that there are several viruses that have been shown to (somehow) be able to pass through, by (a) either unravelling themselves, and passing the virion through as a linear molecule, or (b) perhaps by modifying the substructural arrangements of the otherwise very tight substructures structures that constitute what we call plasmodesmata. Clearly, their function must be related to their structural simplicity or structural complexity. This is what fascinates us about them! So small, so finite yet so difficult to resolve! Thus is can be argued that their ‘gating ability’ will in part, be due to the substructural arrangement within the plasmodesmata themselves. The more complex the substructure, the tighter the control or gating may become. Conversely, the simpler they are the less complex the gating ability, and the wider becomes the range of small molecules which may pass through them. In addition, it is clear that there are many forms and structures visible within plasmodesmata. Interesting debate goes on, concerning the level of complexity, related to (a) evolution of the species; (b) ‘needs’ within cells and particular tissues. Click HERE to go back to the INDEX 4. Length of ‘functional lifetime?’ Again a question which as yet has no single answer. One could argue that during the course of development, there exist times when a distinct ‘need’ to transport substances exists, whist at others, it may be important that the same substances (or related molecules) are not. Here, an example is that of the potato tuber. During development of the tuber, it acts as a strong local sink for carbohydrates, which usually enter as sucrose, and are metabolized to starch. As the potato tuber looses sink capacity during the latter part of the growing season, so transport towards that particular sink will slow down, and may well even be prevented, by gating the plasmodesmata (simply shutting them, cannot be excluded here). Other local sinks may strengthen as a result. The formation of meristematic regions from which new vegetative plant material may sprout, is a visible manifestation of the reversal of the storage capacity and biochemistry of the potato tuber. In the early stages of re-growth, it becomes a strong source, and material that is stored, becomes re-mobilized and redistributed in the growing vegetative plant. Plasmodesmal functionality will have to be re-instated and they will have to act in a reverse role, allowing the passage of substances from the tuber towards the rapidly developing vegetative plant body. Clearly, the concept of ‘gating’, reversibility and functionality are difficult and will continue to be the source of debate and experiment for some time to come. Click HERE to go back to the INDEX 5. What is their fine structure? To most of the biologists that work within the area called cell communication, plasmodesmata are objects of fascination - so small, that their finer details are close to the very limits of resolution with available methods of observation - posing many problems in connection with their mode of formation, development, differentiation, frequencies of distribution and possible function. One of the first studies relating to the ultrastructure of plasmodesmata, was a report in Nature, in the mid-1970’s by AW Robards, who is credited as being the first "plasmodesmologist" to attempt to define, using the electron microscope, the general structure of a plasmodesma. Click HERE to go back to the INDEX
Fig.1. Diagram illustrating the component parts of a plasmodesma. Redrawn, after Robards and Lucas (1990) Click HERE to go back to the INDEX Robards reported in the Nature paper, and in subsequent review articles, that even though plasmodesmata were substructurally complex, but that they appeared to have a number of features of commonality across species.
In some instances, neck constrictions or sphincters have been observed. In many of the species which have these, sphincter-like structure occur on both sides of the plasmodesmata, or only on one side. The grasses typically, have a wide range of structural plasmodesmal forms, with sphincter like structures visible usually in cell walls at the mesophyll-bundle sheath; or bundle sheath-mestome sheath interfaces. Other workers have reported variously that the ER :-
If the ER becomes condensed (appressed) then the question which the use of this term raises, is appressed to what? The term appressed was first applied by Esau, in her description of the ER sheets, which occur in many species of dicotyledonous phloem sieve element,. Here, the ER sheets are truly ‘appressed’ to the plasmamembrane. A great deal of debate continues concerning the structure-function relationships of plasmodesmata. Clearly, there are evolutionary aspects, which tempt us to speculate that "simple" plasmodesmata occur in "more simple plants" but, this is as in all biological science, not strictly true! Take for example the members of the Charales. They are generally believed to be one of the more important direct lines of evolution of the higher land plants. Some authors have stated that the plasmodesmata of the Charalean plasmodesmata reflect their simple origins by virtue of the absence of any substructural details, such as a desmotubule, or central rod. Others argue, that these structures most certainly are there but that we just cannot see them using present fixation techniques. Some recent work, using high pressure frozen material and digital image enhancement studies have suggest some alternative detail structure in plasmodesmata.
Following on the evolution of plasmodesmata, the plant body is in essence, composed of two major compartments, the symplasm and the apoplasm. The apoplasm being the non-living part, the cell walls, whilst the symplasm being the living part or cytoplasm. Unlike the animal cell in which full use is made of the apoplasm and its elaborations to transport intercellular fluids such as blood and lymph fluid, plant cells are by necessity constrained in their ability to make maximal use of the apoplasm. The apoplasm itself has a low carrying capacity. Plasmodesmatal connections can be thought of as allowing for both diffusion and mass flow in directions which can be specified by concentration gradients or by osmotic or hydrostatic forces generated within the symplasm. The may be regulated by a number of interactive processes, in which the formation of callose (B 1,3-glucan) is a predominant factor.
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